The oldest known human DNA found yet shows there may have been another, formerly unknown, human ancestor.

Scientists found the 400,000-year-old DNA in the fossilized thighbone of an early hominid from the “bone pit” at Sima de los Huesos, a unique cave site in Northern Spain. The DNA is four times older than previously discovered samples.

What they found is that the DNA is related to the mitochondrial genome of Denisovans, extinct relatives of Neandertals who lived primarily in Asia, not Europe. The two hominins are believed to have split about 700,000 years ago.

Denisovan remains are extremely rare with the only known fossils being a finger bone and a tooth found in Siberia.

“The fact that the mtDNA of the Sima de los Huesos hominin shares a common ancestor with Denisovan rather than Neandertal mtDNAs is unexpected since its skeletal remains carry Neanderthal-derived features," says Matthias Meyer of the Max Planck Institute for Evolutionary Anthropology.

Considering their age and Neanderthal-like features, the Sima hominins were likely related to the population ancestral to both Neanderthals and Denisovans. This could mean there was another group of hominins brought the Denisova-like mtDNA into the Sima hominins or their ancestors.

The Sima site has yielded the world’s largest assembly of Middle Pleistocene hominin fossils. To date, at least 28 skeletons, have been excavated and pieced together over the course of more than two decades by a Spanish team of paleontologists.

The scientists detailed their findings in the December 5 issue of the journal Nature.